1. Field of the Invention
The invention relates generally to the field of photography, and particularly to cameras. More specifically, the invention relates to apparatus in a camera for preventing an exposed filmstrip returned to a self-thrusting cassette from being re-advanced out of the cassette to double-expose the filmstrip.
2. Description of the Prior Art
In conventional 35 mm film manufacturers cassettes, such as manufactured by Eastman Kodak Co. and Fuji Photo Film Co. Ltd., the filmstrip is wound on a flanged spool which is rotatably supported within a cylindrical shell. A leading or forwardmost portion of the filmstrip approximately 21/3 inches long, commonly referred to as a "film leader", normally protrudes from a light-trapped slit or mouth of the cassette shell. One end of the spool has a short axial extension which projects from the shell, enabling the spool to be turned by hand. If the spool is manually rotated in an unwinding direction, the film roll inside the shell will tend to expand radially since the inner end of the filmstrip is attached to the spool, and the fogged leader portion protruding from the slit will remain stationary. The film roll can expand radially until a non-slipping relation is established between its outermost convolution and the inner curved wall of the shell. Once this non-slipping relation exists, there is a binding effect between the film roll and the shell which prevents further rotation of the spool in the unwinding direction. Thus, rotation of the spool in the unwinding direction cannot serve to advance the filmstrip out of the shell, and it is necessary in the typical 35 mm camera to engage the protruding leader portion to draw the filmstrip out of the shell.
A 35 mm film cassette has been proposed which, unlike conventional film cassettes, can be operated to automatically advance the filmstrip out of the cassette shell by rotating the film spool in the unwinding direction. Moreover, the film leader is non-protruding, i.e. it is located entirely within the cassette shell. Specifically, in U.S. Pat. No. 4,423,943, granted Jan. 3, 1984, there is disclosed a film cassette wherein the outermost convolution of the film roll wound on the film spool is radially constrained by respective circumferential lips of two axially spaced flanges of the spool to prevent the outermost convolution from contacting an inner wall of the cassette shell. The trailing end of the filmstrip is secured to the film spool, and the forward or leading end of the filmstrip is slightly tapered to allow it to freely extend from between the circumferential lips and rest against the shell wall. During unwinding rotation of the film spool, the leading end of the filmstrip is advanced to and through a film passageway opening in order to exit the cassette shell. As a result, all that is needed to advance the filmstrip out of the cassette shell is to rotate the film spool in the unwinding direction. However, according to U.S. Pat. No. 4,423,943, the film cassette is intended to be loaded in a camera only after the non-protruding leader is advanced to the outside of the cassette shell. In the patent, it is suggested that one manually rotate the film spool relative to the cassette shell until the film leader can be manually grasped and attached to a film advancing device in the camera. Thus, the camera is not used to rotate the film spool to advance the filmstrip from the cassette shell.
In conventional 35 mm film manufacturers' cassettes, after the filmstrip is completely exposed, the film spool is rotated in a winding direction to rewind the film leader into the cassette shell. Since the film leader cannot subsequently be advanced out of the cassette shell because of the binding effect between the film roll and the shell, this usually serves as an indication that the filmstrip is substantially exposed. Conversely, in the film cassette disclosed in U.S. Pat. No. 4,423,943, the film leader can be automatically advanced out of the cassette shell by rotating the film spool in the unwinding direction. This can be done regardless of whether the filmstrip is unexposed, substantially exposed or only partly exposed. Thus, the film cassette provides no indication as to the exposure status of the filmstrip, and therefore it might be re-loaded into a camera to double-expose the filmstrip.